1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Prior Art
In the prior art, in order to form a multi-color image by the electrophotography, for example, a series of copying steps of charge, exposure, development and transfer are repeated for each color component to transfer toner images in individual colors onto a copy paper in a superposed manner. For example, electrostatic latent images are formed separately at the respective steps by separated colors such as blue, green and red separated through a color separation filter and are developed in Yellow, Magenta and Cyan and, if necessary, black toner to form toner images. These toner images are laminated on and transferred to a recording paper to form the multi-color image. In this multi-color forming method, however, there arise difficulties: (1) a transfer to a transfer member becomes necessary at the end of development of each color to enlarge the size of machinery and to elongate the time period for the image formation; and (2) warranty for precision against failure of registration due to the repeated operations becomes necessary.
Therefore, there has been proposed a multi-color image forming method for eliminating those difficulties by developing a plurality of toner images on a common photosensitive member in a superposed manner to reduce the transfer steps to one. However, this method is also encountered by a trouble that a toner image obtained at a previous developing step is disturbed at a subsequent developing step or that toner in a developer at a preceding step is mixed with a developer at a succeeding step to disturb the color balance of the multi-color image.
In order to avoid this trouble, there has also been developed a method for forming a multi-color image by adopting a method in which a bias having a superposed a.c. component is applied to a developing device at second or later developments to fly toner onto an electrostatic latent image formed on a photosensitive member. According to this method, the developer layer will not rub the toner image or images formed at the preceding step so that no image disturbance will occur.
The principle of this image forming method will be described in the following with reference to the flow chart of FIG. 4. FIG. 4 shows changes in the surface potential of the photosensitive member and takes up a case in which the charge polarity is positive. Reference letters PH indicate an exposed portion of the photosensitive member; letters DA an unexposed portion of the photosensitive member; and letters DUP the rise of potential, which is caused as a result that positively charged toner T sticks to the exposed portion PH at a first development.
The photosensitive member turning at a constant speed is charged evenly by a scorotron charger to have a constant positive surface potential E, as shown in (a). Next, a first image exposure is effected by means of an exposing source such as a laser, a cathode ray tube or an LED so that the potential of the exposed portion PH drops, as shown in (b), in accordance with the quantity of light. An electrostatic latent image thus formed is developed by means of a developing device to which is applied a positive bias substantially equal to the surface potential E of the unexposed portion. As a result, as shown in (c), the positively charged toner T sticks to the exposure portion having a lower potential to form the first toner image T. The region formed with that toner image has the potential rise DUP as a result of the stick of the positively charged toner T, but will not have the same potential as that of the unexposed portion DA. Next, the photosensitive member surface formed with the first toner image is subjected to a second charge by a charger so that it takes the uniform surface potential E despite whether the toner T is present or absent. This is shown in (d). The surface of that photosensitive member is subjected to a second image exposure to form an electrostatic latent image (as shown in (e)), and a positively charged toner image T' in a color different from that of the toner T is developed like the step (c) to form a second toner image. This is shown in (f). The process thus far described is repeated to form a multi-color toner image on the photosensitive member. The multi-color toner image is transferred onto a sheet of recording paper, and it is further heated and pressed for fixing to obtain a multi-color recorded image. In this case, the photosensitive member is cleaned through cleaning of the toner and charges having remained on its surface and is used for forming a next multi-color image. On the other hand, there is another method by which a toner image is fixed on a photosensitive member in a different manner.
In the method described with reference to FIG. 4, it is desirable that at least the developing step of (f) be conducted such that the developer layer is out of contact with the photosensitive member surface.
Incidentally, in the aforementioned multi-color image forming method, in case that charging is repeated each time of the image formation, a charge eliminating step by a light exposure or a corona discharging may be incorporated before the charging step. On the other hand, the exposure source for the image exposure may be identical or different at each time.
In the aforementioned multi-color image forming method, the four color toners, e.g., Yellow, Magenta, Cyan and black are frequently superposed on the photosensitive member for the following reason. According to the color subtractive principle, a black image must be formed by superposing the three primaries, i.e., Yellow, Magenta and Cyan. However, since the practical toners for the three primaries do not have ideal absorptive wavelength ranges and since the toner images in the three primaries will come out of registration, those three primary toners will find it difficult to reproduce the clear black color required for letters or lines, and in addition the color image is liable to come short of density. As has been described above, therefore, the multicolor image is formed in the four colors composed of the black in addition to the three primaries.
As the latent image forming method for forming the multi-color image, there can be used in addition to the aforementioned electrophotographic method a method, in which charges are implanted directly into an image forming member by means of a multi-styrus electrode to form an electrostatic latent image, or a method in which a magnetic latent image is formed by means of a magnetic head.
In case various colors are to be reproduced by the aforementioned methods, there are the following two systems:
(1) a system in which toners in different colors are not superposed directly; and PA0 (2) a system in which the different-color toners are superposed.
According to the former system (1), as shown in FIG. 14(A), a color reproduction is falsely effected on the recording paper by distributing toners T.sub.1 and T.sub.2 in a non-superposed manner on an image retainer. According to the latter system (2), the color reproduction is effected by developing on a toner image in a certain color a toner in different color in a superposed manner.
Incidentally, in the case of the electrophotographic method, for example, according to the system (2), the light is absorbed by the previously developed toner T to fail to reach the photosensitive layer of the image retainer sufficiently so that the latent image is not formed completely. This invites a tendency that the sticking amount of the later developed toner T.sub.2 becomes less, as shown in FIG. 15 or FIG. 16. According to the system (1), on the other hand, it is necessary to register the image exposures so strictly that the toner images in the individual colors are not registered in identical positions with one another. If the positioning of the image exposure is imprecise, as shown in FIG. 14(B), the tendency is that the toner image T.sub.1 at the preceding step will shield a portion of the image exposure so that the sticking amount of the toner image T.sub.2 to be developed at the succeeding step will become short, as shown in FIG. 14(C). These tendencies indicate that the recording characteristics will become different in dependence upon the spectroscopic sensitivity of the image retainer, the spectroscopic characteristics of the light source for the image exposure, the spectroscopic transmissivity characteristics of the toners, and the order of the colors to be developed.
Incidentally, in the state that the image is formed on the recording paper in the aforementioned manner, the toners having failed to be transferred to the recording paper are left on the image retainer (or the photosensitive member), and this photosensitive member has to be cleaned because those residual toners obstruct the formation of a next image. This cleaning is effected, as will be described hereinafter, by the method in which a blade and/or a fur brush is brought into abutment against the photosensitive member to scrape off the residual toners left on the photosensitive member.
This cleaning exerts a serious influence upon the turning velocity of the photosensitive member. If the blade or the fur brush is brought into abutment against the photosensitive member, more specifically, it acts as it were a brake to fluctuate the turning velocity of the photosensitive member although slightly. If the blade or the fur brush in that abutting state is brought away from the photosensitive member, on the other hand, the turning velocity of the photosensitive member is also fluctuated.
Especially if the r.p.m. of the photosensitive member changes even slightly when the latent images are to be formed, the images obtained have their individual colors misregistered.
In the cleaning device to be used in the color image forming apparatus, moreover, the operations of bringing the blade member into and out of abutment are conducted for the turning period of the image retainer so that the turning velocity of the image retainer is fluctuated to some extent by the frictional resistance resulting from that abutment. As a result, if the blade member is brought into or out of the abutment especially while a latent image is being formed on the image retaining surface by an exposure, the circumferential speed of the image retaining surface may be changed to cause disturbances due to the discontinuity of the image scanning thereby to deteriorate the image quality.